1. Field of the Invention
This invention relates to novel polyimides which exhibit a good heat resistance, high mechanical strength and a low coefficient of linear expansion and are thus adapted for use as substrates such as finely patternized flexible printed board substrates. The invention also relates to processes for preparing such polyimides as mentioned above.
2. Description of the Prior Art
It is known in the art that polyimide resins have very good resistances to heat and chemicals and very good electric and mechanical characteristic properties along with other good characteristics.
A typical polyimide is one, for example, set forth in Japanese Patent Publication No. 36-10999, which is obtained from 4,4'-diaminodiphenyl ether and pyromellitic dianhydride. This polyimide has flexing ether bonds in the main chain thereof and is thus flexible irrespective of the fact that it consists of a totally aromatic polyimide. In contrast, the polyimide is low in elastic modulus, undesirably great in coefficient of linear expansion and poor in dimensional stability.
In recent years, however, there is a demand for the development of polyimide resins which have better thermal dimensional stability along with good mechanical strength in the fields such as of finely patternized flexible printed boards. Currently employed polyimide resins exhibit a coefficient of linear expansion undesirably as large as about 3.times.10.sup.-5 /.degree.C., with the attendant problem that the thermal dimensional stability is so poor that when laminated with metal sheets, they are apt to warp or curl.
On the other hand, there is an increasing demand for the development of polyimide resins which have better thermal dimensional stability and good mechanical strength. In order to meet the demand, extensive studies have now been made. In fact, many attempts have been made to use two or more aromatic diamines so as to improve the mechanical strength and thermal dimensional stability. However, in any of these attempts, it has not been possible to satisfy both requirements for the thermal dimensional stability and mechanical characteristics of polyimide resins.